Conventional tantalum sputter targets manufactured by casting and thermomechanical processing display alternating bands of the (200) and (222) crystallographic orientations across a sputter target's face and through the target's cross section. For example, the target's surface often exhibits alternating lighter shaded grains near the (200) orientation and darker shaded grains near the (222) orientation. Unfortunately, the (200) texture bands have a lower sputter yield than the (222) texture bands that result in substrates having tantalum sputter layers of varied thicknesses within a substrate. For example, sheet resistance uniformity of sputtered films for the 49 point test typically ranges between 2 and 3 percent 1 sigma (or one standard deviation). These non-uniform tantalum layers can cause manufacturers to increase sputtering thickness to eliminate thin regions. Furthermore, the resulting variation in tantalum thickness often precludes the use of sputtered tantalum films for demanding applications.
In the past, manufacturers have used grain growth techniques to control grain orientation. For example, Dunn et al., in U.S. Pat. No. 3,335,037, disclose a rolling/grain recrystallization method for producing tantalum foil. This method maximizes the (110) orientation to produce the tantalum foil—this product's thickness and grain orientation are each unacceptable for sputter target manufacturing.
Zhang, in U.S. Pat. No. 6,193,821, discloses a thermomechanical process for deforming a pure tantalum billet into a sputter target. This process first relies upon side-forging or side-rolling and then upon upset forging or upset rolling. The process produces sputter targets having a predominantly (222) texture and a grain size of less than 25 μm. This process does not appear to eliminate all banding effects associated with targets formed from cast tantalum targets or align the (222) texture in a direction normal to the sputter target's face.
In another thermomechanical process, Michaluk et al., in Pat. Pub. No. WO 00/31310 disclose a process for casting and processing tantalum sputter targets. This process appears to produce tantalum blanks having a primary (222) texture in a direction normal to the sputter target's face with minimal (200)–(222) texture banding throughout the thickness of the blanks. (This specification refers to directions with the convention that for a body-centered cubic (bcc) lattice, h+k+l must equal an even number for the reflection to have a non-zero intensity, the bcc lattice gives rise to a systematic absence when h+k+l is odd. For example, this specification refers to (222) and (200) directions rather than the (111) and (100) directions referenced by other conventions.) Unfortunately, controlling the (222) and (200) directions alone does not provide the necessary grain orientation or control required for the most stringent sputtering applications.